The invention relates to an electronic weigher with a weighing scale located above the weigher casing and supported, via a connecting piece, by a load receiver positioned within the weigher casing, the load receiver being part of a power measuring system.
In such weighers with a weighing scale located above the weigher casing, there is provided, for the connection between weighing scale and load receiver, generally in the upper side of the casing, a round hole whose diameter is somewhat larger than the diameter of the connecting piece between weighing scale and load receiver, since it is necessary to take into account the tolerances of the usual two or more piece casing as well as the tolerances of the power measuring system mounted therein. The relatively wide gap thus produced reduces the protective function of the casing against external air drafts and against dust deposits on the measuring system. Furthermore, the interior of the casing of an electronic weigher is, on account of the heat due to energy losses of the electronic structural elements, somewhat warmer than the environment. This may produce in the aforementioned gap, through a chimney effect, an airflow which passes directly along the weighing scale and thus falsifies the weighing result. This effect appears especially in very precise weighers which indicate a milligram or even fractions thereof. It is particularly disturbing in weighers which are provided, around the weighing scales, with a more or less airproof wind protection member which is opened for the purpose of loading the weigher. When the wind protection member is open, a state of flow is produced that differs from that at a closed condition, so that the reaction upon the weighing scale is also different and different data may result, depending on whether the wind protection is open or closed.
Furthermore, it is known to improve the protection of an electronic weight measuring system from external influences by shaping the gap between the connecting piece or pieces between the weighing scale and the load receiver on the one hand and the casing on the other hand as a labyrinth. In this structure, several short cylindrical elements are fastened, concentrically and with changing diameter, alternately to the casing and to the weighing scale. This produces, to be sure, a good dust protection, but the aforementioned disturbing influence of the heated air ascending from the interior of the weigher casing persists, since the aerodynamic resistance of the labyrinth, to be sure, diminishes the velocity of flow of the air, but on the other hand the flowing air transmits a vertical impulse upon the weighing scale at each deflection within the labyrinth. Besides, a labyrinth presents the disadvantage that the air cushion prevailing under the weighing scale is enlarged, so that at temperature differences between this air cushion and the surrounding air the lifting forces upon the weighing scale become stronger.
The purpose of the invention consists therefore in developing an electronic weigher with a weighing scale located above the weigher casing in such a way that the weighing scale with its connection to the load receiver inside the weigher casing is better produced against the influence of airflows and air temperature differences.
According to the invention, this is achieved by means of a structure wherein below the weighing scale and at a short distance therefrom a lower plate is fastened to the weigher casing, furthermore that the lower plate is provided in its center with a hole through which the connecting piece between weighing scale and load receiver passes with a slight lateral play, and that the lower plate is shaped in such a way that between the lower plate and the weigher casing a path with low resistance remains open for air flow between interior and exterior.
By the insertion of the lower plate the air cushion below the weighing scale is reduced to a minimum. Furthermore, because of the lower plate the flow path of the air between the interior of the casing and the exterior is divided into the paths; one path with high resistance to flow through the narrow gap between lower plate and connecting plate between weighing scale and load receiver, and through the small interspace between weighing scale and lower plate, and a second path with low resistance to flow which passes along the lower plate the the weigher casing. The larger part of the air will therefore flow through the path of less resistance to the flow and thus will not come into contact with the weighing scale. The narrow gap between the lower plate and the connecting piece between weighing scale and load receiver is possible in spite of the tolerances of the structural parts and their assembly since the lower plate is adjustably fastened to the casing.
The lower plate should suitably be larger in area than the weighing scale in order to keep the air flowing along between its lower plate and the weigher casing safely away from the weigher scale. This is accomplished with particular success if the lower plate bears an upright collar which is arranged to closely encircle the weighing scale. In addition, because of this upright collar, air flows from the environment of the weigher are deflected away from the weighing scale and affect the latter to a lesser degree. The upright collar rests advantageously upon the lower plate and can be removed therefrom. Thereby it becomes easier to remove the weighing scale, e.g., for travel purposes. Furthermore, apertures are advantageously provided between the upright collar and the lower plate which permit a discharge of liquid that might have entered between the weighing scale and the upright collar. To prevent this liquid which may have entered from passing into the interior of the weigher casing, the lower plate is advantageously provided on its top side with a ring-shaped bulge around the central hole.
Moreover, it is suitable to adapt the top side of the lower plate in its shape to the bottom side of the weighing scale. Thus, the weighing scale and lower plate can be manufactured in the same deep-drawing mold. This does not cause additional molding cost, and the remaining air volume between weighing scale and lower plate becomes particularly small.
When it is especially desirable for the weighing scale to be protected against airflows coming from the outside, it is feasible to provide, as an advantageous further development in addition to the lower plate, a ring in the form of a truncated cone or a cylinder around the lower plate and the weighing scale.